Thanks to m, Cupcake girl282, candyland7, ValeriaDaughterofApollo, bella 14, Purplemania,
1) So I looked up the definitions of Doors of Death and House of Hades in the back of MoA. I underlined some things you should note "House of Hades: An underground temple in Epirus, Greece dedicated to the Hades and Persephone, sometimes called a necromantion. or "oracle of death." Ancient believed it marked one entrance to the Underworld, and pilgrims would go there to commune with the dead." Did the House of Hades move west with Mount Olympus? Epirus is approximately 136 miles west of Mount Olympus. From New York, that will get you near Pittsburgh. "Doors of Death: A well-hidden passageway that when open allows souls to travel from the Underworld to world of mortals" So the Doors of Death aren't really doors. They are passageways, probably filled with monsters. It is also well hidden. And heavily guarded. 608 pages is starting to sound short with so much that can happen.
Sorry this was so late, but I ran out of predictions until last night. Remember to R&R!